Dual gene control for erythrocyte antigen H-2.7(G): A determinant on complement component C4d

An H-2 variant arising from an intracistronic chromosomal exchange within C4 genes is described. The variant chromosome expresses normal H-2(b) alleles at K, I, and D regions. We propose that the C4 genes are comprised of β1 and α2 genes fromS(b) parent and the γ1 gene from S(f) parent. This variant chromosome, when in heterozygosity with H-2(k) chromosome (β2, α1, γ2), can express H-2.7 (by trans-gene complementation [α1, γ1] a determinant on C4d fragment) on the erythrocyte membrane as well as synthesizing normal levels of C4. These results suggest that C4 deficiency in mice may be caused by juxtaposition of incompatible genes coding for the alpha and the gamma chains of C4 on the same chromosome by intracistronic chromatid exchange rather than by a defect in the structural gene controlling synthesis of C4.